You should tell her of how wonderful she is and add more phrases like how caring she is and how you like the way she carries herself. If you don't really love her and don't see any future for your relationship, then it is better to tell her the truth so as to avoid any relationship complications.
There is not a thing I would change about you and if she says why say her eyes her eyes make the stars look like there not shining her hair her hair falls perfectly without her trying she's so beautiful and i tell her everyday! sorry couldn't help it, but searously tell her she beautiful!!!
Say what you love about her maybe sonething like this: you are the most beautiful woman in the world and i am so lucky to even see you, you are beautiful sweet lively and everything you are is everything i need....i love you, babe